Flash - Drawing Custom Polygons In Box2D?

I'm making a Flash game, and I've encountered a really weird problem while trying to draw a polygonal shape in Box2D.

Here's the code I use:

var fixtureDefs:Array = new Array();
...
var fDef:b2FixtureDef = new b2FixtureDef();
fDef.density = 0;

[code]....

Where vertexArray is a valid array containing 4 b2Vec2 vertices, making up a convex shape.The problem is, when I test, collisions don't work right for that body. Most other objects -enemies, user-controlled characters - pass straight through, as if the body isn't there at all. Some raycasts pass through as well.

Infuriatingly enough, one kind of bodies I have (a custom enemy) somehow does detect the body and collides with it. The raycasts that particular kind of enemy attempts do work - when your character hides behind the polygon, it's like they can't see him.The other weird thing: when I try the same code, only go for SetAsBox instead of SetAsArray, it works exactly as it should.I'm using a custom ContactListener class, but I haven't done any contact filtering (unless it's possible to do that without realising).I'm using the Flash 9 version of Box2D 2.1a.